Conventionally, there has been known an ultrasonic testing method using an ultrasonic array probe in which transducers are linearly arranged to detect a flaw in a wheel such as a railway wheel.
Unfortunately, for a flaw existing near a surface of a test object, flaw echo is buried in surface echo, so that the detection of flaw may be difficult.
Also, there has been an ultrasonic testing method for detecting a flaw near a surface by controlling the beam diameter of the ultrasonic beam of an ultrasonic array probe so that the beam diameter (d) and the in-water wavelength (λ0) of ultrasonic beam are 1/(d·λ0)≧1 (for example, refer to Patent Literature 1) to enhance the defect detectability (S/N). Unfortunately, also in such a method, for a flaw existing near a surface, flaw echo is buried in surface echo, so that the flaw may not be detected sufficiently.